Multistage process relating to the alkaline digestion of raw cellulosic materials for the production of pulp with a high content of alpha cellulose



7 1959 H F. J. wENzL 2,874,044

MULTISTAGE PROCESS RELATING TO THE ALKALINE DIGESTION OF RAW CELLULOSIC MATERIALS FOR THE PRODUCTION OF PULP WITH A HIGH CONTENT OF ALPHA CELLULOSE I Filed Dec. 25, 1955 NATION A IMPRECNATION gg go c GREEN LIQUOR NaOH lOOg/l H HEAT 7 A I \F \J' j v REACTION POINT v A EXTRACTION LIQUOR k EXTRAC"I'ON H 130 170C 130 170C v g gg' NaOH) 100/2 g/l LIQ HEAT Jr 4 I WATER LIQUOR BLACK 105 110C 7 DISPLACEMENT v LIQUOR Ty I 7 WATER WASHING EVAPORATION 100 MELTING PURIFIED A PULP v v DISSOLUTION FLOW SHEET. CAUST CIZING MULTISTAGE PULPING PROCESS. I

GREEN LIQUOR United States Patent MULTISTAGE PROCESS RELATING TO THE AL- KALINE DIGESTION 0F RAW CELLULOSIC MATERIALS FOR THE PRODUCTION OF PULP A HIGH 'CONTENT OF ALPHA CELLU- Hermann Franz Joseph Wenzl, Lueerne, Switzerland Application December 23, 1955, Serial No. 555,169

2 Claims. (Cl. 92-13) This invention relates to an improved process for the production of pulp with high content of alpha-cellulose, by alkaline treatment of vegetable raw materials. Such kind of pulp with high content of alkali-resistant cellulose is of great technical importance especially for the production of artificial fibers, cellulose derivatives e. g. cellulose esters and cellulose ethers, and for the production of high grade papers with special properties.

The object of the invention is the production ofpulps with high content of alpha-cellulose by alkaline treatment, whereby the vegetable raw material in consequence of a series of different stages of treatment will be transformed to the desired degree of a high alpha grade pulp in an economic and most effective form.

A further object of the invention is the execution of the process of pulping in a manner, that the continuous treatment of the cellulosic material With the difierent alkaline solutions, necessary and for use to obtain a high grade alpha pulp, takes place in the same digester, whereby recovery of these alkaline solutions for reuse takes place.

A further object of the invention provides a manner of the execution of the process in that the non-cellulosic components of the vegetable raw material and in the alkaline solutions soluble reaction products are quickly and completely removed from the remaining pulp, in order to obtain the desired high content of alphacellulose in the pulp.

These and further objects of the invention will be realized by the new process, which consists of a series of single stages of treatment under fixed conditions and will be described in relation to the appending flow sheet in a preferred example. It is obvious that this process within the specified conditions and in adaptation to the relative vegetable raw material will undergo certain modifications within the scope of the invention.

The vegetable raw material, which may be wood, straw, grass, bagasse, or any other vegetable fibrous raw material, will be placed in a digester and covered with an alkaline solution. This alkaline solution has e. g. a concentration of active alkali corresponding to a concentration of 100 grams of caustic soda per liter. In adaptation to the specific cellulosic raw material this alkaline impregnation liquor may contain certain other chemical ingredients as e. g. sodium sulfide, sodium sulfite, sodium hyposulfite, sodium carbonate or sodium bicarbonate.

In relation to the invention the term alkaline impregnation liquor may be understood therefore in this enlarged sense. The treatment of the raw material with the alkaline solution takes place at a temperature below 100 C., and normally during two hours, whereby temperature and time of treatment depend on the conditions of the raw material and especially on its degree of humidity. They may be determined by preliminary tests.

Initially, the penetration of the alkaline solution into the air-diy raw material, e. g. wood, takes place by capil- 2,874,044 Patented e 17, rose 'ice larity, whereby the air in the interstices of the raw material otters a remarkable resistance to the penetra tion of the alkaline solution. It will, therefore, be advantageous to accelerate the escape of the air in the interstices by steaming or evacuating the content of the digester. Under the influence of the alkaline solution and after a short while, the cell-walls of the raw material begin to swell and the further impregnation through them and into the interstices takes place only by diffusion. Raw material in wet condition, e. g. floated or steamed wood, contains the cell-walls in the swollen condition, the interstices filled with water. Under such conditions the penetration of the alkaline solution takes place exclusively by diffusion.

So far described, the velocity of impregnation plays an important role in the impregnation of the raw material with the alkaline solution. This velocity depends essentially on the difference of concentration of the alkaline solution inside and outside of the raw material. This invention, therefore, provides the use of an impregnating solution with a high content of caustic soda and. a solution with at least grams of active alkali per liter. Solutions of this concentration cause a high swelling of the celulosic raw material, they loosen the cellulose-hemicelluloselignin linkages and, under certain conditions, transform the cellulose into a cellulosealkali-compound. The velocity or" diiiusion depends further on on the temperature of the impregnating solution and it is afurther object of this invention to keep the temperature during the stage of impregnation high enough to the result of a fast penetration without running the risk that the reaction between the alkaline solution and the non-cellulosic components of the raw material will begin before the impregnation will be completed. Normally, a temperature under 100 C. will be suitable for this purpose.

The first stage of the process will be terminated when the raw material is completely and uniformly impregnated with the alkaline solution. The surplus of the impregnating solution will be drained ofi, the digester contains at that time only the impregnated raw material.

Immediately after the impregnating solution has been drained off, the second stage or" the process will be initiated. It consists in raising the temperature of the impregnated material to the thermal reaction point at which the reaction of the lignin and the other noncellulosic components with the alkaline reagents begins. This raising of the temperature may be achieved by a liquor or by vapor, e. g. by introducing 0t liquors, preheated nearly to the reaction point, which may consist of water, salt solutions, alkaline solutions, or by introducing steam. An especially advantageous form of raising the temperature of the impregnated material provides the heating of the same by exposing it to an electric field of high frequency.

The use of liquors as heat transferring medium involves, of course, the possibility of a certain re-ditfusion of the impregnated alkaline solution into the surrounding aqueous heat transferring medium. In order not to decrease the concentration of alkali in the impregnated raw mate rial before reaching the thermal reaction point, it will be necessary to impregnate in the first stage with an alkaline solution of a higher content of alkali. Using an alkaline solution as heat transferring medium the concentration of this solution may be normally half the concentration of the impregnating alkaline solution in regard to its content of active alkali.

As soon as the temperature of the impregnated raw material has reached the thermal reaction point, the reaction between the alkali and the non-cellulosic compounds of the raw material takes place. Lignin and non-cellulosic compounds become soluble in the alkaline solution. The situation of the thermal reaction point depends reasonably on the one hand on the chemical composition of the raw material, its content of lignin and hemicelluloses, on the other hand on the concentration and effectiveness of the alkaline reagents in the impregnated solution. It must be taken in consideration that the different non-cellulosic components, such as lignin, hexosans and pentosans, ditfer widely in view of their reaction point. As a fact of experience it may be said that the thermal reaction point, according to the process of this invention, lays somewhat lower in comparison with the normal soda or sulfate pulp process. To obtain pulps with high content of alpha-cellulose a maximum temperature of 160 to 170 C. for softwood, 150 to 160 C. for hardwood, and 140 to 150 C. for straw and grasses will be sufficient.

The second stage of the process is terminated as soon as the reaction of the non-cellulosic components with the alkaline solution has taken place. As a matter of fact the third stage of the process begins at the moment of reaching the thermal reaction point. This third stage of the process is the extraction of the now soluble reaction products from the cellulosic residue. This extraction will be facilitated by surrounding the cellulosic residue with an extracting solution, into which the soluble noncellulosic compounds will pass by diffusion. This redifiusion will be so more effective and faster, the greater the gradient of concentration of soluble compounds between the inside of the cellulosic residue.

So far in the second stage of this process and for reaching the thermal reaction point an aqueous solution has been used as a heat carrier, the solubilization and the re-difiusion of the soluble reaction products take place practically at the same time. In this case the concentration of the extraction solution, in regard to its content of active alkali, should be approximately half the concentration of the solution used as a heat carrier.

Using steam as a heat carrier, or exposing the impregnated material to an electric field of high frequency for heating purpose, a preheated alkaline solution must be added as an extraction medium for the soluble noncellulosic components. In this case the concentration of the extraction solution should be approximately half the concentration of the impregnation solution. During the period of extraction the temperature in the digester should be near to the temperature of the thermal reaction point. The extraction stage will be continued until the concentration of the reaction products inside and outside the cellulose residue will be equal or an equilibrium will be obtained. At this point the third stage of the process is terminated.

The temperature in the digester will now be decreased to 105 to 110 C. The extraction solution will be displaced by water of the beforementioned temperature which may contain very little quantities of alkali in order to avoid precipitation of the soluble but still highmolecular reaction compounds. This operation will be continued until the essential part of extraction solution has been removed from the cellulosic residue. A final washing in the usual manner will terminate the process.

The equipment used in this process is the normal equipment ofa soda or sulfate pulp mill and is no part of this invention. Using a normal digester with forced circulation for this purpose, it will be advisable to provide for storage tanks which will serve to prepare and to preheat the necessary solutions for impregnation, heat transfer, extraction and the final washing operation. These storage tanks should be connected with the digester by pipe-lines and pumps, suitably equipped with heatexchangers in order to transfer heat to the solutions in use or to recover heat from hot solutions after use. Exchange of the solutions of the different stages and of difierent concentration shall be performed by displacement and not by dilution.

The recovery of the spent alkaline solutions may be performed in the usual manner of a soda or sulfate pulping process. The high alkaline solution from the first stage (impregnation) can be re-concentrated by fresh liquor from the recovery plant and immediately re-used for impregnation. The solution from the second stage (reaction) will be delivered to the recovery plant, for evaporation, melting and causticizing. The spent liquor of the third stage (extraction) goes likewise to the recovery plant. The very dilute solution of the fourth stage will be used in part for the dissolution of the smelt. The total alkali consumption of this process is not higher than the alkali consumption of a normal soda or sulfate pulping process, although pulps with much higher content of alpha-cellulose may be obtained.

The chemical figures of pulps from beech, obtained by this process, are e. g. as follows:

In comparison, pulps obtained by the normal sulfate process show figures as follows:

Beech, Straw,

percent percent Alpha-Cellulose -80 78-82 Pentosan 10-19 10-15 Solubility in 5% NaOH 10-13 10-15 What I claim is:

1. A multistage pulping process in order to obtain a pulp with a high content of alkali-resistant alpha-cellulose, from vegetable fibrous raw material such as wood, straw, grass, bagasse and similar material, comprising impregnating said raw material with an alkaline solution containing at least grams per liter of active alkali, at a temperature not exceeding 100 C. during impregnation, draining a surplus of the said alkaline solution of the impregnated raw material, heating said impregnated raw material to a reaction temperature ranging from 140 to 180 C., displacing the impregnated alkaline solution at this temperature by an alkaline solution of nearly half the concentration of the impregnating liquor, extracting the non-cellulosic components of the raw material by said alkaline solution and by means of ditfusion until the concentration of the alkali inside and outside of the raw material has reached an equilibrium, decreasing the temperature of the raw material to a range of to C. only when extraction is completed, displacing the alkaline extraction liquor by very dilute alkaline reacting fluid and finally washing the remaining cellulose residue.

2. A multistage pulping process according to claim 1, in which the heating of the impregnated raw material to the reaction temperature is carried out by exposing the said raw material to an electric field of high frequency.

A References Cited in the file of this patent UNITED STATES PATENTS 1,857,432 Codwise May 10, 1932 1,880,044 Richter Sept. 27, 1932 2,061,205 Olsen Nov. 17, 1936 2,228,349 Feldman Jan. 14, 1941 2,269,985 Olsen Ian. 13, 1942 

1. A MULTISTAGE PULPING PROCESS IN ORDER TO OBTAIN A PULP WITH A HIGH CONTENT OF ALKALI-RESISTANT ALPHA-CELLULOSE, FROM VEGETABLE FIBROUS RAW MATERIAL SUCH AS WOOD, STRAW, GRASS, BAGASSE AND SIMILAR MATERIAL, COMPRISING IMPREGNATING SAID RAW MATERIAL WITH AN ALKALINE SOLUTION CONTAINING AT LEAST 100 GRAMS PER LITER OF ACTIVE ALKALI, AT A TEMPERATURE NOT EXCEEDING 100*C. DURING IMPREGNATIION, DRAINING A SURPLUS OF THE SAID ALKALINE SOLUTION OF THE IMPREGNATED RAW MATERIAL, HEATING SAID IMPREGNATED RAW MATERIAL TO A REACTION TEMPERATURE RANGING FROM 140 TO 180*C., DISPLACING THE IMPREGNATED ALKALINE SOLUTION AT THIS TEMPERATURE BY AN ALKALINE SOLUTION OF NEARLY HALF THE CONCENTRATION OF THE IMPREGNATING LIQUOR, EXTRACTING THE NON-CELLULOSIC COMPONENTS OF THE RAW MATERIAL BY SAID ALKALINE SOLUTION AND BY MEANS OF DIFFUSION UNTIL THE CONCENTRATION OF THE ALKALI INSIDE AND OUTSIDE OF THE RAW MATERIAL HAS REACHED AN EQUILBRIUM, DECREASING THE TEMPERATURE OF THE RAW MATERIAL TO A RANGE OF 105 TO 110*C. ONLY WHEN EXTRACTION IS COMPLETED, DISPLACING THE ALKALINE EXTRACTION LIQUOR BY EVERY DILUTE ALKALINE REACTING FLUID AND FINALLY WASHING THE REMAINING CELLULOSE RESIDUE. 